Avoid potential server side request forgeries (SSRFs)

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Metadata

ID: php-security/tainted-url-host

Language: PHP

Severity: Error

Category: Security

CWE: 918

Description

Server side request forgery (SSRF) is a type of exploit where an attacker abuses the functionality of a server to send HTTP requests to an arbitrary domain. SSRFs are dangerous because they can allow an attacker to bypass access controls, such as firewalls, to interact with internal resources.

The rule is important because it protects your application from potential security vulnerabilities. It restricts the ability of potential attackers to trick your server into making requests to arbitrary URLs, which could lead to unauthorized access to sensitive data or systems.

To avoid SSRF vulnerabilities, always sanitize user inputs that will be used in URLs. One way to do this is by using PHP’s built-in filter_var function with the FILTER_SANITIZE_URL option. This will remove any illegal URL characters from the input. Additionally, avoid using user input directly in the construction of URLs. Instead, use a base URL that you control, and append sanitized user input to it. For example, use $base_url = 'https://www.domain.tld/'; and $path = filter_var($_GET['url'], FILTER_SANITIZE_URL); to create a safe URL.

Non-Compliant Code Examples

<?php
function foo() {
    $url = 'https://' . $_GET['url'] . '/path';
    return perform_req($url);
}

function bar() {
    $url = "https://{$_REQUEST['url']}/path";
    return perform_req($url);
}

function baz() {
    $url = sprintf('https://%s/%s/', $_COOKIE['foo'], $path);
    return perform_req($url);
}

Compliant Code Examples

<?php
function foo() {
    $base_url = 'https://www.domain.tld/';
    $path = filter_var($_GET['url'], FILTER_SANITIZE_URL);
    $url = $base_url . $path . '/path';
    return perform_req($url);
}

function bar() {
    $base_url = 'https://www.domain.tld/';
    $path = filter_var($_REQUEST['url'], FILTER_SANITIZE_URL);
    $url = "{$base_url}{$path}/path";
    return perform_req($url);
}

function baz() {
    $base_url = 'https://www.domain.tld/';
    $path = filter_var($_COOKIE['foo'], FILTER_SANITIZE_URL);
    $url = sprintf('%s%s/%s/', $base_url, $path, 'path');
    return perform_req($url);
}